316 SCIENCE PROGRESS. 



be kept in solution. When precipitated the gold cyanide 

 can afterwards be recovered by treatment with KCy. It is 

 clear therefore that the solutions must be circulated, and 

 also that the more free cyanide is present the greater is the 

 percentage of gold precipitated on the cathode. 



The value of electrical precipitation of gold from cyanide 

 solutions has in no case been more clearly demonstrated 

 than in the treatment of slimed ore, and it may fairly be 

 claimed that the problem of its treatment, which for the 

 past three years has taxed the ingenuity of the chemists 

 and engineers of the Rand, could not have been solved 

 without the aid of Siemens and Halske's process. 



When the Rand ore is crushed in stamp batteries, part 

 of it is reduced to an excessively fine state of division, 

 which settles very slowly in still water, and if allowed to 

 remain in the tailings prevents leaching in large vats from 

 being carried on effectively by forming impervious layers in 

 the sand. It was found necessary therefore in South Africa 

 to separate the slimes from the rest of the tailings by 

 running the pulp from the batteries into large vats full of 

 water, when the coarser particles of ore sink and the slimes 

 overflow at the top. The slimes amount to about 30 per 

 cent, of the dry weight of the ore, and though their assay 

 value is not high, nevertheless when thousands of tons of 

 them were run to waste every day the aggregate loss was 

 very great. The problem of the extraction of the gold 

 from this material was therefore one of great importance. 

 It was attacked by many able metallurgists in the Trans- 

 vaal, and the economic success of the treatment now 

 adopted as the result of their labours will have a con- 

 siderable effect on the future prosperity of the gold-field. 

 The method of treatment is as follows 1 : — 



The slimes as received from the mill are suspended in 

 water, of which they form about 2\ per cent. Milk of lime 

 is added to coagulate the fine ore so as to assist it to settle, 

 and part of the water is then got rid of by Spitzkasten 

 followed by settlement in continuous-overflow pits. The 

 settled slimes, containing about 50 per cent, of water, are 



Address by Chas. Butters, Journ. Chem. and Met, Soc. of South Africa, 

 Feb., 1898. 



